In various electrically measuring instruments, test leads are needed to couple to the measuring instrument so as to connect to a DUT (device under test) for measuring. Typical test lead includes two terminals, one terminal coupled to a DUT (device under test), and another terminal inserted into receptacles of the electrically measuring measurement so that physical quantities, for example voltage, current and power, measured by the DUT (device under test) are transmitted to the electrically measuring instrument through the test lead in order to measure for acquiring the data of the physical quantities. Generally speaking, an electrical measuring instrument has multiple receptacles used to measure different physical quantities. For example, one set of the receptacles is employed during a voltage measuring and another set of the receptacles is used during a current measuring. Because impedance of the measurement circuitry connected to the current measurement receptacle is very low, applying a large voltage to the current measurement receptacle may result in an extremely high currents flowing through the current measurement circuitry thereby causing the electrically measuring instrument being damaged or destroyed.